GRPC instrumentation doesn't extract from B3 multi propagator

[alokp]

What version of OpenTelemetry are you using?

0.19.0

What version of Node are you using?

14

Please provide the code you used to setup the OpenTelemetry SDK

const provider = new NodeTracerProvider();
registerInstrumentations({
    instrumentations: [new GrpcInstrumentation()],
    tracerProvider: provider
});
const exporter = new ZipkinExporter({
    serviceName: serviceName,
    url: config.get("zipkin_exporter")
});
provider.addSpanProcessor(new BatchSpanProcessor(exporter));
provider.register({
    propagator: new B3Propagator({
      injectEncoding: B3InjectEncoding.MULTI_HEADER
    })
});
trace.setGlobalTracerProvider(provider);

What did you do?

I am running a GRPC server. My client sends zipkin tracing headers (x-b3-traceid, x-b3-spanid and x-b3-sampled)

What did you expect to see?

I was expecting the B3Propagator to extract the tracing info from the passed headers.

What did you see instead?

Tracing headers are not extracted.

Additional context

My hunch is that this is caused by the TextMapGetter passed by the GRPC instrumentation to B3Propagator. B3Propagator looks for the single b3 header to decide on which extractor to use.

extract(context: Context, carrier: unknown, getter: TextMapGetter): Context {
  if (getter.get(carrier, B3_CONTEXT_HEADER)) {
    return this._b3SinglePropagator.extract(context, carrier, getter);
  } else {
    return this._b3MultiPropagator.extract(context, carrier, getter);
  }
}

Here B3Propagator is expecting TextMapGetter to return a falsey value if the header is not present (defaultTextMapGetter returns undefined in this case) but the TextMapGetter instance passed by GRPC instrumentation returns an empty array instead.